The subject matter of the present invention consists in a ternary active brazing based on a zirconium-nickel alloy composed of 70 atom % to 85 atom % zirconium and 15 atom % to 30 atom % nickel, preferably for brazing ((aluminum-)oxide-)ceramic parts or single crystals or metal parts or for soldering ((aluminum-)oxide-)ceramic parts to single crystals or ((aluminum-)oxide-)ceramic parts or single crystals to metal parts, which contains titanium in addition to the zirconium-nickel alloy.
Active solders are alloys containing at least one element having an affinity for oxygen, such as titanium, so that no fluxes are necessary during soldering.
EP-A-332 978 discloses a brazing foil for brazing ceramic to ceramic, ceramic to metal, and metal to metal which is made of a binary alloy composed of zirconium and nickel. The phase diagram of the zirconium-nickel alloy is described in a standard work by M. Hansen, "Constitution of Binary Alloys", 2nd Edition, New York, 1958, pages 1062/1063.
Experiments conducted by the inventors have shown, however, that the products made using the prior art brazzing foils do not have satisfactory properties, particularly if they are alumina (=aluminum-oxide)-ceramic parts.
When investigating the causes and looking for improvements, the inventors discovered to their surprise that zirconium-nickel alloys are especially suited for brazing or joining ((aluminum-)oxide-)ceramic parts or single crystals or metal parts or for soldering ((aluminum-)oxide-)ceramic parts to single crystals or ((aluminum-)oxide-)ceramic parts or single crystals to metal parts if titanium is added. In this manner, the coefficients of thermal expansion of these novel active solders can be very well adapted to those of the parts to be soldered, and an optimum can be achieved between wetting, mechanical strength, and thermal expansion in the area of the soldered joint, so that stresses between the active-solder layer and, e.g., the ceramic in sensitive components can be minimized or completely avoided.
As was found by the inventors, the brazed joint produced by means of such active brazings not only is high-vacuum-tight but also has a very high mechanical strength.
Especially suited is a ternary active brazing composed of 70 atom % to 85 atom % of the zirconium-nickel alloy and 15 atom % to 30 atom % titanium.
The zirconium-nickel alloy advantageously has a near-eutectic composition, whereby a range near to the eutectic composition (=24 atom % nickel, 76 atom % zirconium) of .+-.5 atom % is to be understood; the eutectic zirconium-nickel alloy is particularly .suited, of course.
An apparatus for fabricating a foil from an active-brazing alloy, particularly from said ternary zirconium-nickel-titanium alloys, by melt spinning which has a uniform thickness and two surfaces that are as smooth as possible comprises a cylindrical crucible made completely of a high-temperature-resistant and highly thermally conductive nonmetallic material, particularly of high-density graphite or of boron nitride, in which the alloy is melted, e.g., by high-frequency heating, and forced through an opening in the bottom of the crucible onto a metal drum of high thermal conductivity rotating at a high circumferential speed, on which the liquid alloy solidifies at a cooling rate on the order of 10.sup.3 to 10.sup.6 .degree. C./s.
The crucible preferably consists of two parts, an upper part and a lower part, which advantageously have a constant wall thickness and are screwed together.
The opening is preferably located at the center of the bottom of the crucible and projects from the bottom surface. Advantageously, the opening is located vertically above the metal drum in the prolongation of the diameter of the drum.
The melt-spinning process serves to fabricate metal ribbons or foils, mostly from alloys, which are brittle if fabricated by a process other than melt spinning. Melt spinning makes it possible to fabricate ductile and, hence, mechanically workable foils, since they are a solidified liquid like glass and, therefore, are also called "metallic glasses".
So far, crucibles of high-temperature-resistant, but poorly thermally conductive nonmetallic material, such as quartz, have generally been used in the melt-spinning process, since melting temperatures between 800.degree. C. and 1500.degree. C. are necessary, depending on the composition of the alloy.
As the inventors have found, such crucibles are not suitable for melt-spinning active-brazing alloys, because such alloys usually wet the crucible, so that, when the active-brazing alloy is pressed through the crucible opening, molten metal will not only solidify in foil form on the metal drum as desired but, because of its wetting ability, will also reach the other portions of the crucible opening and solidify there because of the poor thermal conductivity of the conventional crucibles. As a result, the active-brazing strip will fray or even break.
By the choice of highly thermally conductive nonmetallic material, particularly high-density graphite or boron nitride, as the material for the crucible, this difficulty, particularly in the production of thin, smooth zirconium-nickel-titanium foils, is overcome.
The above-mentioned excellent properties of the ternary active brazing are achieved by the reaction of the active brazing with the ceramic, and become particularly apparent in a pressure sensor comprising a substrate and a diaphragm of ceramic, particularly alumina ceramic, preferably with a purity of 96 wt. %, which are thermally joined
around the periphery in a defined spaced relationship and parallel to each other by means of one of said active brazings e.g., by means of a preform made therefrom, thus forming a chamber.
Such pressure sensors, e.g., resistive or capacitive ones, are typical stress-sensitive components; they should exhibit no or only negligible temperature hysteresis of electrical characteristics in a temperature range of, e.g., -40.degree. C. to +130.degree. C. If the coefficients of thermal expansion of the ceramic and the active brazing are not sufficiently close together, the active brazing will be plastically and, thus, irreversibly deformed during operation in this temperature range, and the ceramic may even come off in one place and another. As a result, the spatial association of the sensor parts will change, even though only slightly. This new configuration, however, results in different electrical characteristics. Since the latter represent the measurand, e.g., a pressure, the sensor now has a (undesired)temperature hysteresis.
To avoid this temperature hysteresis, the active brazing must thus have a coefficient of thermal expansion which is ideally equal to that of the ceramic or the single crystal, i.e., which actually comes as close as possible to the latter coefficient.
In the above-mentioned pressure sensors, the high static strength and high fatigue strength under alternating stresses of the active brazings based on a zirconium-nickel-titanium alloy can be readily proved: In creep tests at elevated temperatures, e.g., 130.degree. C., and under rated pressure and in overload impact tests, no changes in sensor data were observed over long test times.
The invention and its further features will now be explained in more detail with reference to the accompanying drawings, in which the construction of a capacitive pressure sensor and parts of a preferred embodiment of the apparatus for producing the active-brazing foil or ribbon are illustrated as preferred embodiments.